STMPE821 datasheet(8/55 Pages) STMICROELECTRONICS

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Capacitive compensation

STMPE821

8/55

ETC WAIT register state a period of time of which, all TOUCH inputs must remain "NO

TOUCH" for the next calibration to be carried out.

CAL INTERVAL states the period of time between successive calibrations when there are

prolonged NO TOUCH condition.

2.1.1

Noise filtering

When the STMPE821 is operating in the vicinity of highly emissive circuits (DC-DC

converter, PWM controller/drive etc), the sensor inputs will be affected by high-frequency

noise. In this situation, the time-integrating function could be used to distinguish between

real touch, or emission-related false touch.

The INTEGRATION TIME and STRENGTH THRES registers are used to configure the time-

integrating function of STMPE821.

2.1.2

Data filtering

The output from the calibration unit is an instantaneous "TOUCH" or "NO TOUCH" status.

This output is directed to the filtering stage where the TOUCH is integrated across a

programmable period of time. The output of the integration stage would be a "STRENGTH"

(in STRENGTH register) that indicates the number of times a "TOUCH" is seen, across the

integration period.

The "STRENGTH" is then compared with the value in "STRENGTH THRESHOLD" register.

If STRENGTH exceeds the STRENGTH THRESHOLD, this is considered a final, filtered

TOUCH status.

In data filtering stage, 3 modes of operation is supported:

Mode 1: Only the “touch” channel with highest STRENGTH is taken

Mode 2: All the “touch” channels with STRENGTH > STRENGTH THRESHOLD is taken

Mode 3: The 2 “touch” channel with the highest STRENGTH is taken

These modes are selected using the FEATURE SELECTOR register.

The final, filtered data is accessible through the Touch Byte register.

Table 3.

Calibration action under different scenarios

Scenario

Touch sensing and calibration action

IMP>CALIBRATED IMP + TVR

Touch sensing and calibration action

No touch, no calibration

IMP<CALIBRATED IMP + TVR

IMP>CALIBRATED IMP + EVR

No touch, no calibration

IMP<CALIBRATED IMP + TVR

IMP<CALIBRATED IMP + EVR

No touch,

new CALIBRATED IMP = previous

IMP>CALIBRATED IMP

CALIBRATED IMP + change in IMP

IMP<CALIBRATED IMP

No touch,

new CALIBRATED IMP = new IMP

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